A “biosensor” has been defined as an analytical device incorporating a biological or biologically-derived sensing element either integrated within or intimately associated with a physicochemical transducer. Biosensors are generally designed to produce either discrete or continuous digital electronic signals that are proportional to a single analyte or a related group of analytes, although the provision of analogue signals should not be excluded.
There are many areas of application for biosensors including for example environmental sensing, chemical production, and food and drink production and preparation. One area of application that has attracted a great deal of interest however is that of medical diagnostics, monitoring, and treatment. The following discussion addresses primarily these medical applications, although it will be appreciated that the problems and solutions considered may also have non-medical applications.
Biosensors that are either implantable or wearable on a patient's skin can provide substantially continuous monitoring of a given condition and offer the prospect of closed loop treatment systems, where treatment is applied in direct response to the monitored values, as well as giving feedback to users and clinicians. For example, proposals have been made and systems produced that inject insulin into a patient's system in response to the detection of a low blood sugar level. Both types of sensor, implantable and wearable, are likely to have their own distinct advantages, and will be used in different circumstances and to monitor and treat different conditions.
A number of factors are likely to be key to the successful development of commercially viable implantable and wearable biosensors. Chief amongst these is the need for low power consumption. Particularly in the case of implantable sensors, battery life must be extremely long, as surgical intervention would be required to replace a battery. In addition to minimising device power consumption levels, consideration has been given to powering devices using the electrochemical reaction of bodily substances, and even using electric and magnetic fields generated by the body (so-called energy scavenging techniques). In the case of wearable sensors that are likely to be disposable, low cost is also a priority.
To provide substantially continuous monitoring of data from implantable and/or wearable sensors it is necessary to transmit data between the sensor and some monitoring and control system using some form of wireless transmission mechanism. As it is important to minimise the power consumption of such sensors, wireless transmission (e.g. via RF signals) must be at low power and therefore of limited range, and as such requires that the corresponding receiving equipment must be situated within close proximity to the transmitting sensor device. This can be achieved by the patient wearing or carrying a transceiver device, for example, in the form of a wireless PDA or smart phone, which can process and display the data received from the sensors and can retransmit this data, at higher powers, over a suitable access network to, for example, a central monitoring and processing computer system. Alternatively, this could be achieved through a network of numerous transceiver devices (or “base stations”) distributed throughout a location where monitoring is of specific importance, such as a hospital, a care home or other facility, and with which the sensors communicate directly. The provision of such a network for receiving the sensor data transmissions removes the need for each individual to carry a transceiver device and provides a means for tracking the location of each sensor within the network.
In order to successfully implement substantially continuous monitoring and tracking of sensors using a network of transceivers or “base stations”, any system must incorporate a method of allowing each sensor to connect with a base station within close proximity and also allow each sensor to move throughout the network and be handed-off from one base station to another.
U.S. Pat. No. 6,441,747 describes a wireless programmable system for medical monitoring that includes a base unit designed to communicate with a plurality of worn biosensor transceivers. Other documents relevant to this field are: IEEE Trans Biomed Eng, vol 35, no 7, July 1988, p 526-532; Diabetes Technol Ther, vol 1, no 3, 1999, p 261-6; Med Eng Phys, vol 18, no 8, 1996 December , p 632-40; US20010041831; and WO2000067633.